1. Field of the Invention
This invention pertains to a method for determining the stress field in a subterranean rock formation for predicting the direction of extension of hydraulic fractures in the formation, which information is useful in the development and production of hydrocarbons and other mineral values.
2. Background
The stimulation of subterranean hydrocarbon bearing formations by hydraulic fracturing and similar fracturing techniques is a well-known process for enhancement of the recovery rate of hydrocarbon fluids and similar mineral values. One important aspect of fracturing processes pertains to the requirement or desirability of knowing the direction in which the principal fracture will extend. Subterranean formations of at least medium depth are typically in a state of triaxial compression with the largest principal stress oriented vertically. There are, thus, maximum and minimum principal stresses whose magnitudes are less than the vertical principal stress.
Under this stress condition, for hydraulic fractures to occur, the pressure of the fluid to be injected into the formation must overcome the pore pressure and the minimum in situ compressive stress which is generally in the horizontal direction. Typically, a vertical two-winged fracture occurs in the formation subject to the above stress field and extends in the same direction as the maximum principal horizontal stress or, in other words, in a direction perpendicular to the minimum in situ horizontal stress.
The direction of fracture extension is important to the placement of production wells which will produce the fluids whose recovery is enhanced by the fracturing and subsequent stimulation processes. Accordingly, the expected direction of formation fractures is very important to the efficient recovery of hydrocarbon fluids and other mineral values.
Several methods have been proposed for determining the direction of the expected extension of a hydraulic fracture including measurement of shifting or tilting of the earth's surface in the vicinity of the fracture utilizing devices known as tiltmeters and devices for recording seismic events resulting from the fracture extension. A presentation entitled "Comparison of Hydraulic Fracture Orientation Techniques" by L. L. Lacy (Society of Petroleum Engineers, Paper No. SPE 13225) discusses some aspects of the abovementioned methods.
Moreover, it is indicated that heating of subterranean rock formations generates acoustic emissions which are dependent on the temperature of the formation and the in situ stress. A presention by B. Johnson, A. F. Gangi and J. Handin entitled "Thermocracking of Rocks Subjected to Slow, Uniform Temperature Changes" (proceedings of the nineteenth U.S. Symposium on Rock Mechanics, Mackay School of Mines, University of Nevada, Reno, Nev., 1978) discusses the relationship between a subterranean formation under compressive stress when subjected to heating and the acoustic emissions generated by such activity.
In accordance with the present invention, the acoustic phenomena resulting from increasing the temperature generally uniformly in a rock formation from a particular location such as a wellbore is measured to determine the direction of maximum and minimum horizontal stress and thus the probable direction of hydraulic fracture extension. Clearly, the determination of the direction of hydraulic or other man made fracture extension in a particular subterranean formation will provide for more precise placement of wells or the conduction of other operations which might be more efficiently or effectively carried out with the knowledge of the direction which such fractures will take or will likely take as a result of the orientation of the principal stresses acting on the formation.